In an optical disc device, tracking servo is carried out for correcting the position of a light beam, based on a tracking error signal indicating the position deviation between the light beam and a recording track, arranged spirally or concentrically on the optical disc, as an optical information storage medium, so that the light beam, illuminated from an optical pickup, will follow a target track at all times.
Up to now, a tracking-off is generally detected by a method consisting in detecting that the level of a tracking error signal indicating the position deviation between the light beam and a track has exceeded a preset positive or negative value.
FIG. 5 shows a typical illustrative structure of a tracking-off detection circuit. Referring to FIG. 5, the tracking-off detection circuit includes a tracking error detection circuit 2, a tracking actuator 4, a tracking servo circuit 3, receiving the tracking error signal from the tracking error detection circuit 2 to control the tracking actuator 4, and a comparator 7 for comparing the voltage of the tracking error signal output from the tracking error detection circuit 2 with a reference voltage VR. The tracking error detection circuit 2 detects a tracking error from two outputs 109 and 110 of a photodetector 1 which detects the reflected light of a light beam illuminated on the optical disc, not shown, to output the resultant tracking error signal. An output of the comparator 7 is a tracking-off signal. The tracking servo circuit 3 receives the tracking error signal, output from the tracking error detection circuit 2, to control the tracking actuator 4. The tracking servo circuit 3 receives the tracking error signal, output from the tracking error detection circuit 2, and actuates the tracking actuator 4 such as to decrease position deviation between a light spot and the track to correct the light spot position.
When a light beam is illuminated from the optical pickup, not shown, there is produced a difference between the light intensity of the reflected light on the left side and that on the right side of the track direction. By capturing this difference in light intensities by the photodetector 1 which comprises at least two separate photodetector elements, not shown, which are provided on left and right sides, it is possible to detect a position deviation signal, that is, tracking error signal 101, of the light beam with respect to the track center, from the respective two outputs 109 and 110 of the two photodetector elements of the photodetector 1. That is, the tracking error detection circuit 2 outputs the difference signal of the output signals 109 and 110, corresponding to the light intensities of the left and right photodetector elements, not shown, of the photodetector 1, as the tracking error signal 101. For example, in the case of a four division optical sensor system, TE=Q+R−(P+S) is found from outputs P, Q, R and S of the four photodetector elements of the photodetector 1 to output TE as a tracking error signal 101. It is noted that the four photodetector elements of the photodetector 1 output Q, R, S and P counterclockwise, with Q and R being for the left side and S and P being for the right side. In the case illustrated, the signals 109 and 110 are (Q+R) and (P+S), respectively.
The tracking actuator 104 of FIG. 5 is configured for causing movement of an objective lens, not shown, of the optical pickup, also not shown, along the left and right directions with respect to a track. Specifically, the light spot is moved by causing movement of the objective lens. With the optical pickup, not shown, an outgoing light beam of a semiconductor laser is collimated by a collimator lens, and the so collimated light beam is focused on an optical disc via a polarizing beam splitter, a 45°-mirror, a quarter wave plate and an objective lens, with a reflected light beam being returned to the polarizing beam splitter and entered via 45° mirror to the photodetector 1.
During the normal operation, tracking may be achieved by a tracking servo loop. There are occasions where a tracking-off is caused due to disturbances in the tracking error signal caused in turn e.g. by vibrations or signal dropout on the disc. In a well-known manner, such a tracking-off may give rise to information recording and/or replaying for a mistaken track.
On the other hand, if a tracking-off is left unattended, tracking servo runaway may be produced. If once the tracking servo runaway should occur due to a tracking-off, there are occasion where, due to heating of the tracking actuator 4, it may take prolonged-standby time until the power supply of the optical disc device may again be turned on after the power supply is turned off.
Thus, in case of a tracking-off, it is necessary to stop the recording and/or replaying operation promptly to return to the former track. For managing such control, it is necessary to detect a tracking-off promptly and accurately.
In the configuration in which the signal level of the tracking error signal exceeds a preset value (reference voltage VR), as shown in FIG. 5, there are occasions where the normal operation free of a tracking-off is erroneously determined to be a tracking-off, due to noise superposed on the tracking error signal, such that the normal operation is unnecessarily interrupted.
For suppressing the noises or disturbances, such a method consisting in passing tracking error signal through a low-pass filter may be used. In this method, the tracking error signal 101, output from the tracking error detection circuit 2 of FIG. 5, is input to a low-pass filter, not shown, for smoothing. The resulting signal is compared by the comparator 7 with the reference voltage VR and, if the output voltage of the low-pass filter is larger than the reference voltage VR, it is determined that a tracking-off has occurred. Although high frequency noises or disturbances may be suppressed by the low-pass filter, high-frequency components of the tracking error signal are suppressed simultaneously. The result is that, in case a laser beam traverses a large number of tracks speedily under e.g. a tracking-off, the tracking error signal, varied speedily, are suppressed, with the consequence that a tracking-off cannot be detected correctly.
By the above reason, it is not possible to use a low-pass filter capable of reliably suppressing the noises or disturbances superposed on tracking error signal.
Hence, the configuration suffers from a problem that erroneous detection of a tracking-off by the noise or disturbances cannot be prohibited completely.
There is also known a configuration in which the tracking error signal is rectified by a rectifier circuit 8 and subsequently passed through a low-pass filter 9 so as to be then compared with the reference voltage VR to detect the tracking-off (see, for example the Patent Document 1).
[Patent Document 1]
JP Patent Kokai Publication JP-A-63-291223 (pages 3 and 4 and FIG. 1)